Snell’s Law Lesson Plan
Chloe Ouimet
Grade Level: Grade 11
Subject: Physics
Topic: Refraction, Snell’s Law, Dispersion
Materials: Pencil, Glass of water (transparent), Board, PowerPoint
presentation, Worksheet
Subject-specific competencies:
Identifies incident rays and refracted rays in a diagram or an actual situation
Measures the angles of incidence and the angles of refraction in a diagram or an
experiment
Defines the index of refraction of a medium as the ratio of the speed of light in
a vacuum to the speed of light in that medium (n = c/v)
Determines, in experiments or mathematically, the indices of refraction of
various media
Explains qualitatively and quantitatively a phenomenon using the Law of
Refraction (n1sinΘ1= n2sinΘ2) (e.g. a straw in a glass of water)
Explains the phenomenon of total internal reflection (e.g. mirage, fibre optics)
Learning objectives:
Students can make observations about a pencil being immersed in water to
learn about refraction
Student can apply Snell’s law in various situations
Student can use Snell’s law to determine the critical angle of a medium
Student can readily explain dispersion of white light
Student understands the concept of infrared and ultraviolet light
Part I – Introduction
Begin my asking students what they have noticed about light travelling through
transparent media
Write down their answers on the board
Possible answers:
Light refracts
Denser media slows down light
Less dense media accelerates light
Reflection occurs as well
Colours of the rainbow are seen
Begin confirming their ideas with Powerpoint presentation and some real-life
examples
Part II – Theory
In a cup of water, put in a pencil
Ask the students what they see
Expected:
The pencil looks bent
The pencil seems to be disconnected
Light is being refracted
Ask the students to imagine a normal being drawn and tell me whether
the pencil seems to bend towards the normal or away
The light is bending towards the normal because light is travelling
from a less dense medium to a denser medium
Begin PowerPoint presentation
Includes:
Examples of refraction, both from a less dense to a denser
medium and from a denser to a less dense medium
Notation for incident and refracted ray
Partial reflection and partial refraction
Snell’s Law
Total internal reflection
Critical angle
Dispersion
Infrared + Ultraviolet light
Part III – Practice
Give students a worksheet to be completed in class or for homework
Do the first questions on the board
During the problem, elicit students’ answers
Ask them what they think the next step is, what equations they need and
how to generally go about a problem
Allow students to do the rest of the worksheet in pairs or small groups
Bending Light with Triangles – Snell's Law
Subject Area Physics: Snell’s Law and Refraction of Light
Age or Grade Middle School (6th-8th Grade)
Estimated Length 45 Minutes
Prerequisite
Knowledge/Skills Basic knowledge of triangles
Description of
New Content Simple trigonometry, Snell’s law, index of refraction
1) The index of refraction is material-dependent and can be
used to identify a certain material
Understanding 2) Working knowledge of Snell’s law
Goals 3) Simple trigonometry
5 mW laser pointer; a container filled with sugar water; a
Materials Needed protractor
Procedure Opener: The best way to introduce this concept is to just
show the demo. It’s really cool and will grab the students’
attention pretty quickly. Begin by holding the container filled
with liquid up and ask if anyone can identify what it is.
Present a skeptical viewpoint – if a student answers “water”,
reply with “How can you be sure? Do you have the guts to
drink it?” Obviously, don’t let any students consume the
liquid, but using the other senses is still an option. Now, turn
on the laser and hold the beam perpendicular to the
container. Please exercise caution when dealing with laser
radiation, regardless of optical power; emphasize to students
that lasers are not toys. Ask the students if any of them can
guess what will happen when the angle changes. Write the
responses on the board. Then slowly change the angle and
observe the beam “bend” as the angle increases.
Picture Time: Have students draw a picture of the scenario
in groups. Have them discuss what they think is happening.
Snell’s Law: Introduce Snell’s Law and the index of
refraction.
Taking Measurements: Have students take several
measurements of various angles and have them calculate the
index of refraction for each one. (Note: this assumes the
index of refraction of air is exactly 1 and that the index of
refraction of the container is negligible.) Have students
calculate the mean and standard deviation for their estimate
and compare as a class.
(Optional) Computer Activity: Have students go online and
look what the material may be. This site has a decent list.
What is the index of refraction?
Evaluation How can Snell’s Law be used to estimate the index of
Questions refraction?
Have students hypothesize whether the index of refraction
will change for various concentrations of sugar water. Split
the class up into groups with different concentrations and
have them present to the class their results. As a class,
Extension Activity combine the data and observe any trends.
References N/A
